首页 > 代码库 > 转载:android笔记--android中的多线程--Handler, Looper, MessageQueue, Message类

转载:android笔记--android中的多线程--Handler, Looper, MessageQueue, Message类

什么时候使用多线程:

1. 耗时操作使用多线程, 耗时操作放在UI线程中会导致用户的操作无法得到响应.

2. 阻塞操作使用多线程, 理由同上.

3. 多核CUP的设备使用多线程, 可以有效提高CPU的利用率.

4. 并行操作使用多线程.

 

android中的多线程模型主要涉及的类有:Looper, Handler, MessageQueue, Message等. 

 一:Looper类:

 1 static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>(); 2     private static Looper sMainLooper;  // guarded by Looper.class 3  4     final MessageQueue mQueue; 5     final Thread mThread; 6 public static void loop() { 7         final Looper me = myLooper(); 8         if (me == null) { 9             throw new RuntimeException("No Looper; Looper.prepare() wasn‘t called on this thread.");10         }11         final MessageQueue queue = me.mQueue;12 13         // Make sure the identity of this thread is that of the local process,14         // and keep track of what that identity token actually is.15         Binder.clearCallingIdentity();16         final long ident = Binder.clearCallingIdentity();17 18         for (;;) {19             Message msg = queue.next(); // might block20             if (msg == null) {21                 // No message indicates that the message queue is quitting.22                 return;23             }24 25             // This must be in a local variable, in case a UI event sets the logger26             Printer logging = me.mLogging;27             if (logging != null) {28                 logging.println(">>>>> Dispatching to " + msg.target + " " +29                         msg.callback + ": " + msg.what);30             }31 32             msg.target.dispatchMessage(msg);//超级重要33 34             if (logging != null) {35                 logging.println("<<<<< Finished to " + msg.target + " " + msg.callback);36             }37 38             // Make sure that during the course of dispatching the39             // identity of the thread wasn‘t corrupted.40             final long newIdent = Binder.clearCallingIdentity();41             if (ident != newIdent) {42                 Log.wtf(TAG, "Thread identity changed from 0x"43                         + Long.toHexString(ident) + " to 0x"44                         + Long.toHexString(newIdent) + " while dispatching to "45                         + msg.target.getClass().getName() + " "46                         + msg.callback + " what=" + msg.what);47             }48 49             msg.recycle();50         }51     }52 public void quit() {53         mQueue.quit(false);54     }55 public void quitSafely() {//最好用此方法56         mQueue.quit(true);57     }

 

Looper类用来创建消息队列. 每个线程最多只能有一个消息队列:Looper唯一的构造方法如下

1 private Looper(boolean quitAllowed) {2         mQueue = new MessageQueue(quitAllowed);3         mThread = Thread.currentThread();4     }

 

android中UI线程默认具有消息队列, 但非UI线程在默认情况下是不具备消息队列的. 如果需要在非UI线程中开启消息队列, 需要调用Looper.prepare()方法, 在该方法的执行过程中会创建一个Looper对象

 1  public static void prepare() { 2         prepare(true); 3     } 4  5     private static void prepare(boolean quitAllowed) { 6         if (sThreadLocal.get() != null) { 7             throw new RuntimeException("Only one Looper may be created per thread"); 8         } 9         sThreadLocal.set(new Looper(quitAllowed));10     }

 

而Looper的构造函数中会创建一个MessageQueue instance(Looper的构造函数是私有的, 在Looper类之外无法创建其对象).  

此后再为该线程绑定一个Handler instance, 然后调用Looper.loop()方法, 就可以不断的从消息队列中取出消息和处理消息了.

 1 public Handler() { 2         this(null, false); 3     } 4 public Handler(Callback callback) { 5         this(callback, false); 6     } 7 public Handler(Looper looper) { 8         this(looper, null, false); 9     }10 public Handler(Looper looper, Callback callback) {11         this(looper, callback, false);12     }13 public Handler(Callback callback, boolean async) {14         if (FIND_POTENTIAL_LEAKS) {15             final Class<? extends Handler> klass = getClass();16             if ((klass.isAnonymousClass() || klass.isMemberClass() || klass.isLocalClass()) &&17                     (klass.getModifiers() & Modifier.STATIC) == 0) {18                 Log.w(TAG, "The following Handler class should be static or leaks might occur: " +19                     klass.getCanonicalName());20             }21         }22 23         mLooper = Looper.myLooper();24         if (mLooper == null) {25             throw new RuntimeException(26                 "Can‘t create handler inside thread that has not called Looper.prepare()");27         }28         mQueue = mLooper.mQueue;29         mCallback = callback;30         mAsynchronous = async;31     }32 public Handler(Looper looper, Callback callback, boolean async) {33         mLooper = looper;34         mQueue = looper.mQueue;35         mCallback = callback;36         mAsynchronous = async;37     }
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(创建handler的时候必须关联一个looper)

Looper.myLoop()方法可以得到线程的Looper对象, 如果为null, 说明此时该线程尚未开启消息队列.

1 public static Looper myLooper() {2         return sThreadLocal.get();3     }

二:Handler: 

Handler类用于处理消息. 该类具有四个构造函数:(如上)

1. public Handler(). 创建好的Handler instance将绑定在代码所在的线程的消息队列上, 因此一定要确定该线程开启了消息队列, 否则程序将发生错误. 使用这个构造函数创建Handler instance, 一般来说, 我们需要重写Hanler类的handleMessage()方法, 以便在之后的消息处理时调用.

2. public Handler(Callback callback). Callback是Handler内部定义的一个接口, 因此想要使用这个构造函数创建Handler对象, 需要自定义一个类实现Callback接口, 并重写接口中定义的handleMessage()方法. 这个构造函数其实与无参的构造函数类似, 也要确保代码所在的线程开启了消息队列. 不同的是在之后处理消息时, 将优先调用callback的handleMessage()方法,返回true,则消息处理结束;返回false。则会调用Handler对象的handleMssage()方法.

1  final MessageQueue mQueue;2  final Looper mLooper;3  final Callback mCallback;4  public interface Callback {5         public boolean handleMessage(Message msg);6     }7  private static void handleCallback(Message message) {8         message.callback.run();9     }
 1 /** 2      * Handle system messages here. 3      */ 4     public void dispatchMessage(Message msg) { 5         if (msg.callback != null) { 6             handleCallback(msg);//如上的方法 7         } else { 8             if (mCallback != null) { 9                 if (mCallback.handleMessage(msg)) {10                     return;11                 }12             }13             handleMessage(msg);14         }15     }

 

3. public Handler(Looper looper). 这个构造函数表示创建一个Handler instance, 并将其绑定在looper所在的线程上. 此时looper不能为null. 此时一般也需要重写Hanler类的handleMessage()方法

4. public Handler(Looper looper, Callback callback). 可以结合2和3理解.

 1  public final Message obtainMessage() 2     { 3         return Message.obtain(this); 4     } 5 private static Message getPostMessage(Runnable r) { 6         Message m = Message.obtain(); 7         m.callback = r; 8         return m; 9     }10 public final boolean post(Runnable r)11     {12        return  sendMessageDelayed(getPostMessage(r), 0);13     }14 public final boolean postAtTime(Runnable r, long uptimeMillis)15     {16         return sendMessageAtTime(getPostMessage(r), uptimeMillis);17     }18 public final boolean postDelayed(Runnable r, long delayMillis)19     {20         return sendMessageDelayed(getPostMessage(r), delayMillis);21     }22  public final boolean postAtFrontOfQueue(Runnable r)23     {24         return sendMessageAtFrontOfQueue(getPostMessage(r));25     }26  public final boolean sendMessage(Message msg)27     {28         return sendMessageDelayed(msg, 0);29     }30  public final boolean sendEmptyMessage(int what)31     {32         return sendEmptyMessageDelayed(what, 0);33     }34 public final boolean sendEmptyMessageDelayed(int what, long delayMillis) {35         Message msg = Message.obtain();36         msg.what = what;37         return sendMessageDelayed(msg, delayMillis);38     }39 public final boolean sendEmptyMessageAtTime(int what, long uptimeMillis) {40         Message msg = Message.obtain();41         msg.what = what;42         return sendMessageAtTime(msg, uptimeMillis);43     }44 //Handler所有post、send方法,最后调的都是此方法...45 public boolean sendMessageAtTime(Message msg, long uptimeMillis) {46         MessageQueue queue = mQueue;47         if (queue == null) {48             RuntimeException e = new RuntimeException(49                     this + " sendMessageAtTime() called with no mQueue");50             Log.w("Looper", e.getMessage(), e);51             return false;52         }53         return enqueueMessage(queue, msg, uptimeMillis);54     }55 //入队,其实调的是MessageQueue的入队方法56 private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {57         msg.target = this;58         if (mAsynchronous) {59             msg.setAsynchronous(true);60         }61         return queue.enqueueMessage(msg, uptimeMillis);62     }63 //Handler的hasMessage、removeCallback是MessageQueue的方法64  public final void removeMessages(int what) {65         mQueue.removeMessages(this, what, null);66     }67  public final void removeCallbacksAndMessages(Object token) {68         mQueue.removeCallbacksAndMessages(this, token);69     }70 public final boolean hasMessages(int what) {71         return mQueue.hasMessages(this, what, null);72     }73 public final boolean hasCallbacks(Runnable r) {74         return mQueue.hasMessages(this, r, null);75     }

 

 三,MessageQueue:

MessageQueue类用于表示消息队列. 队列中的每一个Message都有一个when字段, 这个字段用来决定Message应该何时出对处理. 消息队列中的每一个Message根据when字段的大小由小到大排列, 排在最前面的消息会首先得到处理, 因此可以说消息队列并不是一个严格的先进先出的队列.

 方法主要有:

Message next() {...}void quit(boolean safe) {        if (!mQuitAllowed) {            throw new RuntimeException("Main thread not allowed to quit.");        }        synchronized (this) {            if (mQuitting) {                return;            }            mQuitting = true;            if (safe) {                removeAllFutureMessagesLocked();            } else {                removeAllMessagesLocked();            }            // We can assume mPtr != 0 because mQuitting was previously false.            nativeWake(mPtr);        }    }//入队 boolean enqueueMessage(Message msg, long when) {...}//判断消息队列中是否包含某个消息boolean hasMessages(Handler h, int what, Object object) {...}//将某个消息从队列中移出去,该消息被回收到消息池中了void removeMessages(Handler h, int what, Object object) {...}

四:Message:

Message类用于表示消息. Message对象可以通过arg1, arg2, obj字段和setData()携带数据, 此外还具有很多字段. when字段决定Message应该何时出对处理, target字段用来表示将由哪个Handler对象处理这个消息, next字段表示在消息队列中排在这个Message之后的下一个Message, callback字段如果不为null表示这个Message包装了一个runnable对象, what字段表示code, 即这个消息具体是什么类型的消息. 每个what都在其handler的namespace中, 我们只需要确保将由同一个handler处理的消息的what属性不重复就可以.

 

 1     public int what; 2     public int arg1;  3     public int arg2; 4     public Object obj; 5     public Messenger replyTo; 6     /** If set message is in use */ 7     static final int FLAG_IN_USE = 1 << 0; 8     /** If set message is asynchronous */ 9    static final int FLAG_ASYNCHRONOUS = 1 << 1;10     /** Flags to clear in the copyFrom method */11    static final int FLAGS_TO_CLEAR_ON_COPY_FROM = FLAG_IN_USE;12     int flags;13     long when;    14     Bundle data;    15      Handler target;        16      Runnable callback;       17     // sometimes we store linked lists of these things18     Message next;19 20     private static final Object sPoolSync = new Object();21     private static Message sPool;22     private static int sPoolSize = 0;23 24     private static final int MAX_POOL_SIZE = 50;
 1 public static Message obtain() { 2         synchronized (sPoolSync) { 3             if (sPool != null) { 4                 Message m = sPool; 5                 sPool = m.next; 6                 m.next = null; 7                 sPoolSize--; 8                 return m; 9             }10         }11         return new Message();12     }13 public static Message obtain(Message orig) {14         Message m = obtain();15         m.what = orig.what;16         m.arg1 = orig.arg1;17         m.arg2 = orig.arg2;18         m.obj = orig.obj;19         m.replyTo = orig.replyTo;20         if (orig.data != null) {21             m.data = http://www.mamicode.com/new Bundle(orig.data);22         }23         m.target = orig.target;24         m.callback = orig.callback;25 26         return m;27     }28 public static Message obtain(Handler h) {29         Message m = obtain();30         m.target = h;31 32         return m;33     }34 ......35 void clearForRecycle() {36         flags = 0;37         what = 0;38         arg1 = 0;39         arg2 = 0;40         obj = null;41         replyTo = null;42         when = 0;43         target = null;44         callback = null;45         data = http://www.mamicode.com/null;46     }47 public void recycle() {48         clearForRecycle();49 50         synchronized (sPoolSync) {51             if (sPoolSize < MAX_POOL_SIZE) {52                 next = sPool;53                 sPool = this;54                 sPoolSize++;55             }56         }57     }58 public void copyFrom(Message o)59 public long getWhen()60 public void setTarget(Handler target)61 public Handler getTarget()62 public Runnable getCallback()63 public Bundle getData() {64         if (data =http://www.mamicode.com/= null) {65             data = http://www.mamicode.com/new Bundle();66         }68         return data;69     }70 public Bundle peekData() {71         return data;72     }73 public void setData(Bundle data)74  public void sendToTarget()

 

将消息压入消息队列: Message对象的target字段关联了哪个线程的消息队列, 这个消息就会被压入哪个线程的消息队列中.

1. 调用Handler类中以send开头的方法可以将Message对象压入消息队列中, 调用Handler类中以post开头的方法可以将一个runnable对象包装在一个Message对象中, 然后再压入消息队列, 此时入队的Message其callback字段不为null, 值就是这个runnable对象. 调用Handler对象的这些方法入队的Message, 其target属性会被赋值为这个handler对象.

2. 调用Message对象的sendToTarget()方法可以将其本身压入与其target字段(即handler对象)所关联的消息队列中. 

 

将未来得及处理的消息从消息队列中删除:

调用Handler对象中以remove开头的方法就可以.

 

从消息队列中取出消息并处理消息: 所有在消息队列中的消息, 都具有target字段. 消息是在target所关联的线程上被取出和处理的.

 1 /** 2      * Handle system messages here. 3      */ 4     public void dispatchMessage(Message msg) { 5         if (msg.callback != null) { 6             handleCallback(msg); 7         } else { 8             if (mCallback != null) { 9                 if (mCallback.handleMessage(msg)) {10                     return;11                 }12             }13             handleMessage(msg);14         }15     }

 

1. 如果取出的Message对象的callback字段不为null, 那么就调用callback字段的run()方法(callback字段的类型是runnable). 注意此时并不开启一个新的线程运行run()方法, 而是直接在handler对象(即Message的target字段)所关联的线程上运行.

2. 如果取出的Message对象的callback字段为null, 且Handler对象中的callback字段也为null, 那么这个消息将由Handler对象的handleMessage(msg)方法处理. 注意Message对象的callback字段是Runnable类型的而Handler对象的callback字段是Callback类型的, Handler对象的callback字段是在创建Handler instance的时候指定的, 如果没有指定则这个字段为null, 详见Handler类的四个构造方法.

3. 如果取出的Message对象的callback字段为null, 且Handler对象中的callback字段不为null, 那么这个消息将由Handler对象中的callback字段的handleMessage方法优先处理.

 

线程间通信: 有了以上的叙述, 线程间的通信也就好理解了. 假如一个handler关联了A线程上的消息队列, 那么我们可以在B线程上调用handler的相关方法向A线程上的消息队列压入一个Message, 这个Message将在A线程上得到处理.